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LXXXIX. On the Pressure on the Poles of an Electric Arc. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



INSERTED in your October issue is a letter from Dr. Ratner 

 in which he objects to the argument of Prof. Duffield 

 that any motion which ions may acquire in the field of an 

 electric arc cannot give rise to a pressure upon its poles. It 

 is not for me to discuss the part of his communication which 

 deals with Prof. Duffield's work. But he goes on to make 

 deductions from experiments upon the electric wind; and as 

 one of the investigators of that subject to whom he refers, I 

 think it is desirable for me to draw attention to the faulty 

 interpretations that he places upon some of the results of 

 their work. 



In his own work which he quotes, the vane upon which he 

 detected a pressure was placed behind a perforated electrode 

 which received the ions. The vane therefore received some 

 of the momentum of the wind which blew through the per- 

 forations without experiencing the compensating attraction 

 of the ions in the discharge; this was taken entirely by the 

 electrode. He is therefore incorrect in stating that those 

 experiments on an electric wind give evidence of a repulsion 

 of an electrode which has an opposite sign to the ions giving 

 rise to the wind. . 



Secondly, in his remarks on the diminution of the wind 

 with high values of current he ignores the fact that an 

 electric current in a gas is not in general carried by one sign 

 of ion only. In discharge at atmospheric pressure from a 

 point or a wire with small values of current, only one sign of 

 ion is present through the greater part of the path of the 

 discharge. But this is not true for all values of pressure and 

 current, and it is very improbable that it is true in arc 

 discharge for any values of pressure and current. 



In conjunction with Mr. H. E. Gr. Beer, I have been 

 engaged for some months on an investigation of the electric 

 wind in the arc under various conditions. We find that in 

 general it is exceedingly minute. Its direction is from cathode 

 to anode, a fact which under certain circumstances is in accord- 

 ancewiththe manometric observations made byDewar many 

 years ago and referred to by Duffield in his paper. The 

 results are quite consistent with the view that the effect is a 

 residual one, being the difference between the effects which 

 the positive and negative ionic streams would have separately 



